This vulnerability allows an app to install any number of apps with
any type of permissions without user's explicit consent. It is based
on two things:

1. You can install an app from Google Play using just the browser,
even from PC.
2. An app can embed a browser and automatically login into your
Google account without any notification, using a few permissions.

Description
------------------------
One can build an Android app, let's call it Trojan, that requires
these permissions:

android.permission.INTERNET - Allows applications to open network sockets.
android.permission.GET_ACCOUNTS - Allows access to the list of
accounts in the Accounts Service.
android.permission.USE_CREDENTIALS - Allows an application to
request authtokens from the AccountManager.

These are the steps to reproduce it:
1. Trojan app contains a WebView that will automatically login into
user's Google account by requesting authtokens from the Account
Manager, user will not be notified nor have any way to stop this.
2. The WebView will load the Google Play web site and inject
JavaScript code on page load.
3. The JavaScript code will make a request to get the device
information and CSRF tokens, it will get information about all devices
registered with that account. Remember the browser is logged in with
user's Google account.
4. Using this information it can issue a request to install ANY app
on Google Play, on EVERY device registered with that Google account.
The user will not be prompted and will not have any way to stop this.

Scenarios
------------------------
- Trojan app could be full screen and the user will not even see the
install notification.
- Trojan app could choose to install the app on a different device,
from the devices registered with the Google account, the user will not
see anything unusual on the current device where the Trojan runs.
- The installed app can have access to ALL permissions (if it
specifies so in the manifest) without the user explicitly approving
that, it could have access to services that cost you money, like
sending SMS or making phone calls, manage accounts, disable your
phone, just look at the permission list.
- Since you have access to all Google data for that account, there
are other scenarios, like accessing emails and more, but the PoC did
not address those.

The fix
------------------------
The Google fix, as far as I could tell, was to not allow the browser
to automatically login. Instead, the user will be prompted with a text
that says it would allow the app to have access to all Google data.
This however does not inform the user that it will allow automatic
installation of any app, potentially causing direct and immediate loss
of money.

I will not release the PoC, I think it would be too easy to cause real
damage. However it is not that difficult to implement.

Vendor contact timeline
------------------------
2013-12-16 - Contact security(at)google.com.
2013-12-17 - Received reply that the issues was passed to
security(at)android.com.
2013-12-20 - Received reply that they could not reproduce the issue.
2013-12-20 - Sent a stripped down version of the PoC, not much different.
2014-01-16 - Request status update.
2014-01-24 - Received response that the rollout of the fix started last week.
2014-02-12 - Received response that the fix is live for 100% users/devices.